Heat exchangers and other devices typically include a stack of bent tubes connected at their ends to flat tubesheets that are welded to another sheet metal product to form a fluid header. The tube ends and flat tubesheets must usually be welded, and the welds must not only provide a joint with sufficient structural strength to join the pieces, but must also provide a water or fluid-tight seal so that the fluid may transfer between the header and the individual tubes without leaking.
In a typical coil for a heat exchanger, there may be more than fifty tubes to be welded to the tubesheet. Considering that there are typically two similar headers for each coil, there may be well over one-hundred welds to be performed. Manual welding of so many joints is time consuming and therefore potentially expensive. However, implementation of robotic welding has been difficult. Given the need for fluid tight seals, it is necessary that each weld completely surround each junction of a tube end and the tubesheet. Given the size of the coils, manipulation of the coil for accurate welding has been problematic, and it is difficult and expensive to attempt to make a fixture that would hold such bulky workpieces repeatedly and consistently in the same position with respect to a welding machine so that such fluid-tight welds could be produced.
Attempts to achieve consistent, reproducible welds in mass production through use of commercially available robotic systems have also met with problems. It has remained difficult to achieve acceptable fluid-tight welds in successive combinations of tubes and tubesheets.